1 Field of the Invention
The present invention relates to an improved fluorescent lamp and, more particularly, to a fluorescent lamp having excellent emission characteristics even if an amount of phosphor coated on a glass bulb is small. 2. Description of the Related Art
A luminous flux of a fluorescent lamp is increased when an amount of phosphor coated on the inner surface of a glass bulb is increased within a predetermined range. When the amount of phosphor is increased, however, cost of the resultant fluorescent lamp becomes high. In particular, the cost is greatly increased in a fluorescent lamp using a rare-earth phosphor.
For example, a high color rendering type fluorescent lamp has an excellent color rendering property and a high luminous flux as compared with a general illumination fluorescent lamp in which a halophosphate phosphor is used. However, the cost of a rare-earth phosphor used in this high color rendering type fluorescent lamp is several tens of times that of the halophosphate phosphor. For this reason, high color rendering type lamps are not popular yet.
An expensive rare-earth phosphor is used in a special-purpose lamp such as an exposure light source for electrophotography or the like.
In order to reduce the cost of such an expensive fluorescent lamp a technique is described in Published Unexamined Japanese Patent Application No. 53-867 wherein a light-emitting layer has a two-layered structure consisting of an inexpensive halophosphate phosphor layer and an expensive rare-earth phosphor layer formed thereon. According to this technique, an amount of expensive rare-earth phosphor is reduced, and an inexpensive halophosphate phosphor is used instead, thereby reducing cost without decreasing the luminance of the lamp. In a fluorescent lamp having this structure, sodium ions free from a glass bulb are adsorbed into the halophosphate phosphor layer directly coated on the glass bulb to prevent a chemical reaction between the sodium ions and mercury in the filling gas, thereby preventing deterioration of the fluorescent lamp. Although this fluorescent lamp is brighter than the fluorescent lamp coated with only the halophosphate phosphor, emission of the halophosphate phosphor excited by ultraviolet rays passing through the rare-earth phosphor causes a change in emission spectrum of the rare-earth phosphor.
FIG. 1 is a graph showing a luminous flux (curve a) of a high color rendering type fluorescent lamp having a halophosphate phosphor layer and a three component type phosphor layer, and a general color rendering index (to be referred to as Ra hereinafter) (curve b). This graph is obtained with a transparent glass tube whose inner surface is coated with:
2.0 g of a halophosphate to form a first layer; and
a three component type phosphor consisting of a mixture of 25 wt % of (Sr,Ca,Ba).sub.5 (PO.sub.4).sub.3.Cl:Eu, 40 wt % of LaPO.sub.4: Ce,Tb, and 35 wt % of Y.sub.2 O.sub.3: Eu.
The above graph also shows measurement results of the luminous flux (curve a) and the general color rendering index (Ra) (curve b) of fluorescent lamps (FL40SS lamps) when the coating amount of the halophosphate phosphor is kept unchanged and the coating amount of the three component type phosphor is changed.
As is apparent from this graph, when the coating amount of the three component type phosphor is reduced, both the luminous flux and the index Ra are reduced. In particular, a decrease in Ra is significant. When a coating amount ratio of the three component type phosphor to the halophosphate phosphor is reduced, an ultraviolet absorbency of the three component type phosphor layer is reduced. The ultraviolet rays having passed through the three component type phosphor layer stimulate the halophosphate layer as the first layer to cause emission of the halophosphate layer.
The high color rendering fluorescent lamp having this structure cannot satisfy all characteristics, i.e., a high color rendering property, a high luminous flux, and low cost of the phosphor. When the phosphor cost is reduced, the luminous flux is reduced and the color rendering property is degraded.
This situation is not limited to the fluorescent lamp coated with the three component type phosphor but is also applicable to a fluorescent lamp having a halophosphate phosphor layer formed between a rare-earth phosphor layer and the glass bulb. For example, in a lamp in which a halophosphate phosphor layer is formed on the surface of, e.g., a Y.sub.2 O.sub.3 :Eu phosphor layer, the coating amount of the Y.sub.2 O.sub.3: Eu phosphor is reduced to realize low cost. In this case, an emission color is changed, and the luminous flux is reduced.